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Exotic forest insects

Exotic pests are one of the more serious threats to forest health. While few exotics pose serious challenges, there are some insects, diseases, and plants that do. Eradication is possible if small infestations can be detected early.

Posted on August 8, 2014 by Bill Cook, Michigan State University Extension

In this article series, I will review the more serious exotic forest pests. Over 40,000 exotic species have been introduced into North America, and most have proven to be useful to humans and benign to the natural environment. Most exotic introductions fail to establish, but some have grown invasive and damaging.

Among the exotic forest insect pests, there are about three dozen important species. However, according to Michigan State University Extension, two borers and two adelgids are of particular note.

The Asian long-horned beetle (ALHB) (Anoplophora glabripennis) has not yet been identified in Michigan; the nearest infestation is near Cincinnati, Ohio and an infestation near Chicago was eradicated. However, there is a large infestation around Worcester, Massachusetts. The USDA maintains an ALHB website with current information about the beetle.

The ALHB is a large and robust beetle, an inch or more in length, black with white spots, and antennae of alternate white (bluish) and black segments. It is larger than our native wood borers, but a couple of our natives might be mistaken for ALHB.

These borers prefer maple species (sugar, red, silver and boxelder) but have a long list of menu choices. Exit holes of the emerging adults are about a half-inch in diameter. The larvae chew their way through the wood of the tree. Weakened branches can fall, damaging property and potentially injuring people.

The emerald ash borer (EAB) (Agrilus planipennis) is now familiar to most Michigan residents and the EAB range includes much of the greater Midwest and Northeastern states. The Michigan Department of Agriculture and Rural Development maintains a quarantine program.

The EAB kills only ash trees (Fraxinus spp.). The half-inch long adult is bright green and might be confused with predatory tiger beetles and common Polydrusus weevils. The larvae form serpentine tunnels under the bark, but do not bore into the wood. The adults emerge from D-shaped exit holes.

The EAB first affects the tops of trees, and then works downward. Decline can be evident over several growing seasons. Woodpecker activity may indicate numerous larvae. The trees will often send up sprouts from the tree base.

The national ash resource is threatened by this exotic beetle. In Michigan, about five percent of the statewide forest volume is ash. More importantly to many, a large share of residential and park trees are ash. Loss of these trees results in significant visual impact. The EAB has been, so far, the most expensive forest pest with damages of nearly two billion dollars.

The hemlock woolly adelgid (HWA) (Adelges tsugae) and balsam woolly adelgid (BWA) (Adelges piceae) appear very similar but damage only their respective tree species. Adelgids are tiny insects that insert a long stylus (feeding tube) into the twig and feed on the tree juices. They cover themselves with a protective white “woolly” coating, sort of like tiny moth balls. The adelgids concentrate near the bases of the needles.

Neither of these adelgids is established in Michigan, although three infestations of the HWA have been eradicated (hopefully). Introductions have been traced to nursery stock from eastern states where the insect has become well-established.

About 30, from over 400, exotic insect species have caused significant forest health problems in North America. More information about Michigan forest pests can be found on the Upper Peninsula Tree Identification website and the Michigan DNR Forest Health website.

Related Articles

March 8, 2018 | Bill Cook | Reporting exotic species observations is a good practice, even though many of us are reluctant to do so. Increased knowledge, for the both the reporter and the recipient, may be the greatest benefit.

Related Resources

David Smitley, Michigan State University Department of Entomology, and Deborah G. McCullough, Michigan State University Departments of Entomology and Forestry | Guidelines for homeowners with hemlock trees infested with hemlock woolly adelgid.